Secondary batteries are widely used as power sources of portable electronic devices such as for OA (office automation), FA (factory automation), household appliances, communication devices and the like. In particular, since the volume efficiency improves when a device is equipped with a lithium ion secondary battery, leading to a decrease in the size and weight of the instrument, portable devices using lithium ion secondary battery are on the rise.
Meanwhile, regarding large secondary batteries, the use of large secondary batteries is under research and development in a number of fields related to environmental issues, such as load leveling, UPS (uninterruptible power supply) and electric car to begin with. Among them, since lithium ion secondary batteries, which are one type of non-aqueous electrolyte secondary battery, have large capacity, high output, high voltage, excellent long term conservation ability and the like, research and development in particular are proceeding in a number of fields.
A separator used in a non-aqueous electrolyte secondary battery, for instance, a lithium ion secondary battery, is a member bearing the role, by being inserted between the positive electrode and the negative electrode, of preventing a short circuit that accompanies a contact between the active materials of both electrodes, and at the same time, of retaining the electrolytic solution to secure electric conductivity. Therefore, in addition to the necessity of being provided with insulation property and a porous structure, a separator used in this type of battery needs to be provided with permeability to air to secure passage of lithium ions; from such necessities, porous films are used in general.
Regarding porous film used in the separator of a non-aqueous electrolyte secondary battery, a producing method for a laminated film of polyethylene and polypropylene has been proposed, for instance, in Japanese Patent Publication No. 2883726, which is hereby incorporated by reference herein in its entirety. The characteristics of this porous film producing method reside on the point of controlling a higher order structure with a high draft rate when the raw sheet is fabricated, and the point of multiple-stage drawing at small ratios at low temperature and high temperature.
In addition, a composite sheet is proposed in Japanese Patent Application Laid-open No. 2006-264029, which is hereby incorporated by reference herein in its entirety, in which two or more layers of thermoplastic polymer porous sheet having a melting point of 200° C. or lower and non-woven fabric sheet having no substantially stable melting point are layered.
In addition, the use of a microfibrous polymer web (non-woven fabric) produced by accumulating a fiber spun over a collector by the electrospinning method is disclosed in Japanese Patent Application Laid-open No. 2002-249966, which is hereby incorporated by reference herein in its entirety, as a separator for lithium secondary battery. Then, from the point of view of improving the strength thereof, the use of a non-woven fabric made of polyimide resin, in which the diameter of the fiber produced by the electrospinning method is 1 μm or less, is proposed in Japanese Patent Application Laid-open No. 2005-019026, which is hereby incorporated by reference herein in its entirety, as a battery separator.
In addition, a separator for lithium ion secondary battery is proposed in Japanese Patent Application Laid-open No. 2006-092829, which is hereby incorporated by reference herein in its entirety, in which a non-woven fabric obtained by the electrospinning method is layered over both sides of a woven sheet surface.
With the recent increase in the capacities of batteries, the importance regarding safety of batteries is increasing. The shutdown property (hereinafter, also referred to as “SD property”) may be cited as a property of a battery separator contributing to safety. This SD property is a property whereby the micropores of the separator become occluded when a temperature is high (130 to 150° C.), resulting the ion conduction within the battery to become blocked, preventing an increase in temperature subsequently inside the battery. In case a porous film is to be used as a battery separator, it needs to be a porous film provided with this SD property.
The breakdown property (hereinafter referred to as “BD property”) may be cited as another property contributing to safety. This BD property is a property demonstrating the heat resistance of the separator, which, while retaining the SD property (that is to say, by retaining the state in which the micropores of the separator are occluded) at a temperature where the SD property is expressed or higher, maintains the shape of the separator up to a higher temperature (150° C. or higher) state, separating the positive electrode and the negative electrode. In case a porous film is to be used as a battery separator, providing this BD property is also important from the point of view of securing safety.
However, regarding existing separators using non-woven fabric, attention to such SD property and BD property was not sufficient. For instance, although the use of porous non-woven fabric as a separator for secondary battery is described in the aforementioned Patent References 3 to 5, no consideration whatsoever is made regarding the SD property and the BD property, which is not sufficient to secure the safety of the battery.